Janus particles refer to a kind of anisotropic particles two hemispherical surfaces of which have different chemical properties. Janus particles have special structures, excellent properties, and broad application prospects. Amphiphilic nano-particles are chemically asymmetrical at both ends and are therefore one of the Janus particles. The methods for producing amphiphilic Janus particles fall into four main categories: selective surface modification, template-directed self-assembly, the use of phase separation, and control of surface nucleation.
Silica raw materials are readily available and inexpensive, making them ideal starting materials for amphiphilic Janus particles. When amphiphilic Janus particles are produced using silica particles as precursor particles, a selective surface modification method is often used. For example, with homogeneous silica beads as precursor particles, the silica particles are dispersed at a paraffin-water interface at a higher temperature, and then the temperature is lowered to solidify the paraffin. The silica particles are thus immobilized on the solidified paraffin-water interface to form small solid paraffin droplets which are then chemically modified on the side which is not covered by paraffin. Subsequently, the paraffin is dissolved in an organic solvent, and the resulting particles can be further chemically modified (Liang H., Langmuir 2006, 22, 9495-9499). In addition, free-radical polymerization can also be initiated through the surface of the liquid-liquid interface. Amphiphilic silica particles were produced by grafting hydrophobic polystyrene and hydrophilic poly(methyl acrylate) on the respective hemisphere of silica (Zhang J., Langmuir 2009, 25 (11), 6431-6437). However, the amphiphilic nano-silica produced by such method has a particle size larger than 100 nm, and the process is complex with small amount produced each time, expensive and difficult to large-scale application.
Due to the large amount of hydroxyl groups on the surface of nano-silica particles, the silica particles have strong hydrophilic properties per se. In addition, the hydroxyl groups on the surface of nano-silica particles easily interact with functional groups such as amino groups and hydroxyl groups to modify the hydrophilicity and hydrophobicity of the surface of the particles through chemical modification. Therefore, in the preparation of amphiphilic nano-silica, the surface of nano-silica can be partially hydrophobized to arrange dispersively the hydrophilic and hydrophobic groups on the surface of silica. Therefore, nano-silica can exhibit some amphiphilic properties. Chinese patent application CN101792147B describes the surfaces of silica particles having a particle size of 100-800 nm are chemically modified with phenyltrimethoxysilane, (4-chlorophenyl)triethoxysilane, (4-chlorophenyl)trichlorosilane or 4-chloromethylphenyltrichlorosilane to change the hydrophilicity and hydrophobicity of the particles, in order to produce amphiphilic silica particles. Chinese patent application CN101428807A uses titanate coupling agents for in-situ modification of nano-silica particles. Organic groups are attached to the nano-silica surface to reduce the number of hydroxyl groups on the surface of the particles and thus reduce the surface energy, to obtain easily dispersible amphiphilic nano-silica. Chinese patent application CN101659415A adopts the same strategy by introducing multi-functional silane coupling agent to modify the silica particles in situ to reduce the surface energy of the particles, so that a nano-dispersion of silica is produced by the subsequent combined dispersion techniques.
However, the amphiphilic nano-silica produced by the above prior art method is subjected to both hydrophilic and lipophilic modifications based on one particle. These two kinds of modification will interact with each other, which limits the types of hydrophilic or lipophilic groups on the particle surface. The number of surface hydrophilic and lipophilic groups cannot be adjusted independently, thereby limiting the adjustment of hydrophilic and lipophilic properties of the amphiphilic nano-silica. Thus the amphiphilic nano-silica particles themselves are not strongly hydrophilic and lipophilic. Also, the existing preparation technique for amphiphilic nano-silica cannot provide amphiphilic nano-silica having a particle size of less than 100 nm. Moreover, the inherent complexity and high cost of the process make it difficult to independently adjust the hydrophilic and lipophilic properties of amphiphilic nano-silica.